flambda-backend: Allow forceful crossing of portability and contention (#2725)

* Add basic tests

* Add -allow-illegal-crossing flag

* Implement for records, variants, and private types, and update tests

* Fix bug causing types to always cross

* Add more tests

* Fix types with abbrev and manifest

* Remove comment about module signatures

* Add more tests for illegal crossing in signatures

* Add tests demonstrating that values can't cross illegally

* Remove no-op annotations

* Treat private types as non-nominative

* Print annotations when there is illegal crossing

* Remove incorrect test

* Add test demonstrating incorrect inference for type parameter

* Fix test numbering

* Fix inference test

* Add cr to remove type_has_illegal_crossings

* Update note about printing jkinds

* Add comment for printing case C1.3

* Fixed incorrect type_has_illegal_crossings value

* Fix test for inferring 'a

* Rename allow-illegal-crossing.ml to allow_illegal_crossing.ml

* Add tests for mode crossing

Author: liam923

driver/main_args.ml

@@ -728,6 +728,10 @@ in
   \    allows a set of extensions, and every successive universe includes \n\
   \    the previous one."
 
+let mk_allow_illegal_crossing f =
+  "-allow-illegal-crossing", Arg.Unit f,
+  "Type declarations will not be checked along the portability or contention axes"
+
 let mk_dump_dir f =
   "-dump-dir", Arg.String f,
   "<dir> dump output like -dlambda into <dir>/<target>.dump"
@@ -920,6 +924,7 @@ module type Common_options = sig
   val _extension : string -> unit
   val _no_extension : string -> unit
   val _extension_universe : string -> unit
+  val _allow_illegal_crossing : unit -> unit
   val _noassert : unit -> unit
   val _nolabels : unit -> unit
   val _nostdlib : unit -> unit
@@ -1201,6 +1206,7 @@ struct
     mk_extension F._extension;
     mk_no_extension F._no_extension;
     mk_extension_universe F._extension_universe;
+    mk_allow_illegal_crossing F._allow_illegal_crossing;
     mk_for_pack_byt F._for_pack;
     mk_g_byt F._g;
     mk_no_g F._no_g;
@@ -1328,6 +1334,7 @@ struct
     mk_extension F._extension;
     mk_no_extension F._no_extension;
     mk_extension_universe F._extension_universe;
+    mk_allow_illegal_crossing F._allow_illegal_crossing;
     mk_noassert F._noassert;
     mk_noinit F._noinit;
     mk_nolabels F._nolabels;
@@ -1421,6 +1428,7 @@ struct
     mk_extension F._extension;
     mk_no_extension F._no_extension;
     mk_extension_universe F._extension_universe;
+    mk_allow_illegal_crossing F._allow_illegal_crossing;
     mk_for_pack_opt F._for_pack;
     mk_g_opt F._g;
     mk_no_g F._no_g;
@@ -1607,6 +1615,7 @@ module Make_opttop_options (F : Opttop_options) = struct
     mk_extension F._extension;
     mk_no_extension F._no_extension;
     mk_extension_universe F._extension_universe;
+    mk_allow_illegal_crossing F._allow_illegal_crossing;
     mk_no_float_const_prop F._no_float_const_prop;
     mk_noassert F._noassert;
     mk_noinit F._noinit;
@@ -1714,6 +1723,7 @@ struct
     mk_extension F._extension;
     mk_no_extension F._no_extension;
     mk_extension_universe F._extension_universe;
+    mk_allow_illegal_crossing F._allow_illegal_crossing;
     mk_noassert F._noassert;
     mk_nolabels F._nolabels;
     mk_nostdlib F._nostdlib;
@@ -1825,6 +1835,7 @@ module Default = struct
     let _no_extension s = Language_extension.(disable_of_string_exn s)
     let _extension_universe s =
       Language_extension.(set_universe_and_enable_all_of_string_exn s)
+    let _allow_illegal_crossing = set Clflags.allow_illegal_crossing
     let _noassert = set noassert
     let _nolabels = set classic
     let _nostdlib = set no_std_include

driver/main_args.mli

@@ -34,6 +34,7 @@ module type Common_options = sig
   val _extension : string -> unit
   val _no_extension : string -> unit
   val _extension_universe : string -> unit
+  val _allow_illegal_crossing : unit -> unit
   val _noassert : unit -> unit
   val _nolabels : unit -> unit
   val _nostdlib : unit -> unit